Many semiconductor packages, whether for individual power devices, large microprocessors or hybrid assemblies of several integrated circuits attached to a single substrate, run hot enough to require a heat sink. Heat sinks are effective because they conduct the heat flux originating in the semiconductor material into regions where there is sufficient surface area to transfer the heat to another medium (generally air) at a rate sufficient to prevent an undesirable rise in temperature in the device to which the heat sink is attached. Since power dissipation (heat) usually originates in a local region that is small relative to the entrance area of the heat sink, internal dispersion of heat to within the heat sink can be improved by including one or more heat pipes inside the heat sink, near where it is attached to the semiconductor package being cooled.
This is all well and good, but as shown in FIG. 1, the entire flux of heat energy still has to transit the path from the semiconductor die 1, through the lid of the package 2 and into the heat sink 3. That path has a cross section whose effective extent is delimited by arrows 4, and that at its start is equal to the area of contact of the die 1 with the inside of the lid 2 of the semiconductor package. To keep the temperature of the die 1 as low as possible it is desirable that cross sectional area of that path 4 increase (by reaching the heat sink 3) before there is too great an increase in temperature at the die 1. Unfortunately, the thermal resistance of that portion of the thermal path from the die 1 (or dies) to the heat sink 3 is often high enough that it becomes the limiting factor for power dissipation, rather than the limit being determined by a heat sink operating at ambient temperature. That is, it can be the case that using a "better" heat sink (such as one that includes a heat pipe 5) does not result in a significant reduction in die temperature for operation at a given power level.
There are at least three reasons for this, and they include the thermal resistance of the (internal) mechanical interface between the die and the inside of the lid, the thermal resistance of the cross section of the lid as the heat flux travels laterally through it to spread out for contact with the larger external heat sink, and the thermal resistance across the (external) mechanical interface of the surface of the lid with the surface of the heat sink. The thermal resistance of the internal mechanical interface can be reduced by the use of thermally conductive compounds. The thermal resistance of the external mechanical interface is affected by the fit and finish of the two parts. Even what appears to be good surface to surface contact can be adversely affected by microscopic features. Even so, the external mechanical interface itself can generally be made a good as needed by proper sizing, machining, and the use of a suitable thermal compound between the parts. That leaves lateral flow of heat flux through the lid of the package as the remaining untamed source of significant thermal resistance. If the thermal resistance of lateral heat flow through the lid could be lowered, then for a particular heat sink configuration a die (or a collection of dies in a hybrid) could operate at higher power for a given temperature or at a lower temperature for a given power. In really high power systems such an improvement may mean the difference between "passive" cooling by convection using ambient air and the need for actual refrigeration.